1. Field of the Invention
The present invention relates to a robot system for successively taking out bulk stacked workpieces of the same type by means of a robot comprising an arm and a hand, and a judgment method for judging a workpiece gripping state by a hand.
2. Description of the Related Art
When conveying a workpiece with a robot comprising an arm and a hand, various photoelectric sensors attached to the hand may be used as means for confirming a gripping state of a workpiece by a hand. In this case, it is preferable to accurately position the hand with respect to the workpiece to prevent a gripped workpiece by a hand from leaving the detection range of a photoelectric sensor. In this respect, in order to successively take out and convey workpieces which are bulk stacked or piled up in a container, it is necessary to pick up and grip workpieces which are laid in arbitrary postures. For this reason, it is not always easy to accurately position a hand with respect to a workpiece. One of the possible ways to confirm the gripping state of bulk stacked workpieces is setting a photoelectric sensor independent from the robot at the outside of the container and then moving the hand move in the detection range of that photoelectric sensor. However, when the confirmation result by the photoelectric sensor indicates that the hand is not gripping a workpiece, it is necessary to move the hand inside the container again so as to take out a workpiece, and therefore the cycle time for the take-up process ends up becoming longer. Note that, one example of a hand for gripping bulk stacked workpieces is an absorption-type hand which comprises a cylinder device attached to an arm, and an absorption attached to a front end part of a piston rod of the cylinder device. Such an absorption-type hand is disclosed in JP-A-H6-344351.
Incidentally, there is also a robot in the prior art which uses a device other than the above-mentioned photoelectric sensor as means for confirming the workpiece gripping state by a hand. For example, JP-A-2013-195199 and JP-A-2013-56402 propose a robot which measure a weight of a workpiece using a force sensor which is arranged between an arm and a hand, and JP-A-2013-27940 proposes a robot comprising a weight measuring device which is attached to a front end part of an arm. Further, JP-A-H8-94424 proposes a weight measuring device comprising a load sensor which is attached to a hand so as to measure a load acting on a mounting plate of a vacuum pad, and JP-A-H4-152223 proposes a handling device which measures the weight of a workpiece using an electromagnetic force type measuring device with a coil and magnet. However, in order to employ the arts which are described in JP-A-2013-195199, JP-A-2013-56402, JP-A-2013-27940, JP-A-H8-94424, and JP-A-H4-152223, it is necessary to incorporate a dedicated measuring device into an arm or hand, and therefore the robot tends to have a complicated structure and thus involve an increased manufacturing cost.
A robot system and judgment method for judging a workpiece gripping state by a hand, using a simple and inexpensive mechanism, have been sought.